1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus, and more specifically relates to an electrophotographic developing device that maintains a stable electrical charge for long-term use and stably provides high-quality images with high density and no fogging, and an electrophotographic image forming apparatus including the electrophotographic developing device.
2. Description of Related Art
In a typical electrophotographic image forming apparatus used in offices, a two-component developer is usually used. A two-component developer is used in a developing device in such a way that a toner and a carrier are agitated and mixed to electrically charge the toner and the charged toner is then provided on the surface of a photoreceptor to visualize (develop) an electrostatic latent image. Thus, only the toner is consumed in a repeated development of electrostatic latent images. The carrier is not consumed and remains in the developing device for a repeated use. In such a conventional method for development, only the consumed toner is replenished into a developing device.
A repeated image formation causes adhesion of toner particles, additives of the toner, and/or components of the toner matrix, which are generated because of mechanical stress in a developing device, on the surfaces of the carrier particles (this phenomenon is known as a “spent toner problem”). This lowers the electrical chargeability of the carrier, i.e., lowers the ability to electrically charge the toner. Thus, a repeated image formation impedes appropriate electrical charging of the toner, causing fogging and low image density of the images, which requires replacement of the developer after the formation of a certain number of images.
The development characteristics of a developer significantly differ between immediately before and after the replacement. Thus, the characteristics of the images also differ largely. Given this, Japanese Patent Application Laid-Open Publications Nos. Sho59-10071 and 2007-079578 each propose a developing device that achieves stable image quality through the prevention of degradation of the developer, the extension of the developer life, and the control of the development characteristics of the developer within certain ranges. In detail, the developing device maintains the charging characteristics of a developer in a developing device over an extended period through a repeated disposal of a small amount of degraded developer having lowered electrical chargeability and a repeated replenishment of a fresh developer, stabilizes the image characteristics, extends the developer life, and lowers the replacement frequency of the developer. Such a development system is known as an auto-refining development system or a trickle development system.
In such a system, a small amount of degraded developer is repeatedly discarded parallel to a repeated formation of images while a fresh developer having high electrical chargeability is repeatedly replenished. In this way, stable electrical chargeability of a developer in a developing device can be maintained for an extended period. As a result, high-quality images with high density and no fogging can be stably produced over an extended period.
A developer containing a carrier having a large specific weight provides a quick rise of electrical charge, in other words, provides required electrical charge at the initial rise or immediately after it is replenished, because it has high agitation intensity in a developing device. However, such a carrier inflicts a large amount of stress on a toner, accelerating the degradation of the carrier due to the spent toner problem.
A decrease in the chargeability due to the degradation of a carrier causes a decrease in the electrical charge of a toner. This causes a decrease in an image quality due to fogging. This may also cause a decrease in the transfer rate from the photoreceptor to a transfer medium and/or adhesion of the carrier.
The decrease in the image quality and transfer rate and the adhesion of the carrier due to the degradation of the carrier can be prevented through an increase in the amount of the carrier supplied, i.e., an increase in the replacement rate. If the amount of the carrier supplied is reduced, the image quality will decrease largely due to accelerated degradation of the carrier.
The copying of a document having a large black area consumes a large amount of toner. If a developer that is a mixture of a toner and a carrier is supplied in copying such a document, the large amount of the toner must be replenished (supplied) together with a large amount of the carrier. As a result, a large amount of the developer is wasted.
Although an auto-refining development system is employed to extend the developer life and stabilize the image quality, the use of a developer containing a carrier having a large specific weight cannot sufficiently achieve these advantages over an extended period because a large amount of the developer is wasted.
On the other hand, Japanese Patent Application Laid-Open Publication No. 2009-205149 discloses a technology that can be applied to a developing device requiring replenishment of the developer. In this technology, a porous magnetic carrier having a small specific weight is used for a carrier contained in a developer in the developing device and is used for a carrier for replenishment. The toner in a developer not used for a long time has a decreased electrical charge. Thus, a porous carrier having a small specific weight cannot sufficiently charge such a lowly charged toner within a short time. This causes drawbacks such as fogging and toner scattering.